"Tricky Dicky" wrote:

That does not sound good, do you live in a former mining area or is the
area prone to sink holes?
Some creaking is not usually an issue but loud cracks suggest further
investigation is needed.

Thanks for your thoughts. No to the mining area and sink holes question.
If there was something serious happening I would have expected it to be
evident visually: cracks in plaster, displacement of the plasterboard panels
in the downstairs ceilings, something on those lines - but there's nothing
(at least nothing so far).